What Is an ACL Tear?

A common sports injury, ACL tears occur more frequently among female athletes.

The anterior cruciate ligament (ACL) is one of the most commonly injured ligaments of the knee.

According to the American Academy of Orthopaedic Surgeons (AAOS), an estimated 200,000 ACL injuries occur annually, and 100,000 ACL reconstruction surgeries are performed each year.

As one of the four primary ligaments of the knee, the ACL connects the front top part of the shinbone to the back bottom part of the thigh bone and keeps the shin from sliding forward. It also provides rotational stability to the knee.

The ACL keeps the knee hinged together and functioning properly with the help of the posterior cruciate ligament (PCL), which like the ACL, is also a crossed ligament in the center of the knee.

There are two additional ligaments on the sides of the knee called the medical collateral ligament (MCL) and the lateral collateral ligament (LCL), as well as multiple muscles, tendons, and secondary ligaments.

What Causes an ACL Tear?

ACL injuries tend to happen more often in people who play high-impact sports where rapid pivoting and turning are common, such as basketball, football, skiing, and soccer.

AAOS reports that about 70 percent of ACL tears happen without direct contact to the knee. In these instances, the tears tend to happen when a person is running and quickly changes direction, suddenly twists or cuts, or if they hyperextend their knee coming down from a jump.

However, a direct hit to the knee, such as during a football tackle, can also cause ligaments to tear.

Female athletes who play pivoting sports are 2 to 10 times more likely than male athletes to sustain a knee ligament injury, such as an ACL injury, according to the AAOS.

This may be due to differences in physical conditioning, muscular strength, and neuromuscular control. It might also be due to pelvis and lower leg alignment, a wider pelvis, or other anatomical differences.

What Happens During an ACL Tear?

Most ACL tears occur in the middle of the ligament or when the ligament is pulled off the thigh bone. The tears form a gap between the torn edges and do not heal on their own.

Damage to the meniscus, articular cartilage, or other ligaments in the knee also occurs in about half of ACL injuries.

An “unhappy triad” is the most common multi-ligament injury — it involves injury to the ACL, MCL, and medial meniscus. Some people also experience bruising on the bone beneath the cartilage surface.

Many people tend to feel or hear a popping in their knee when they injure their ACL. They may also experience a brief hyperextension of the knee joint causing the knee to bend backward.

After the injury, most people have to stop the activity they were doing, and the knee usually swells for a few hours.

Types of ACL Sprains

There are different severities of ACL sprains, which are categorized in the following way:

Grade I sprain: Pain with minimal damage to the ligaments, but still able to help keep the knee joint stable.

Grade II sprain: More ligament damage and mild looseness of the joint, often called a partial tear.

Grade III sprain: The ligament is completely torn and the joint is very loose or unstable.

Can ACL Tears Occur in Children?

In 2008, the Journal of the American Academy of Orthopaedic Surgeons reported that most ACL injuries occur in those between the ages of 15 to 45 because this group has an active lifestyle and greater participation in sports.

However, other studies show a rise in the number of reported ACL tears in children, with the injury seen in kids as young as age two.

The true incidence of ACL tears in children is not known, and it isn’t clear if the increase is related to kids being more active in sports such as soccer, or other factors like increased awareness by physicians and better diagnostic techniques.

ACL Injury Prevention

While some ACL injuries are not preventable, the following may help minimize the risk:

Strength training to develop strong thigh and hamstring muscles

Maintain a stretching routine with your legs

Thoroughly warming up before playing sports

Learn to land on the balls of your feet rather than flat-footed

When skiing, ensure your ski bindings are set correctly so that your skis release when you fall

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